Ride and handling performance pose conflicting requirements on vehicle suspensions and thus designs are generally driven to achieve an acceptable compromise between the two, depending on the application. While a stiffer suspension is essential to enhance the rollover threshold value and control performance of the vehicle, a softer suspension yields good ride performance at the expense of poor stability and directional control.
In beam axle suspensions, because both wheels share the same axle, vertical movement of one wheel causes a similar movement in the other wheel. Both wheels respond as one unit, which detracts from the ride quality of the vehicle.
Independent suspension systems have a number of advantages over beam axle suspensions. The independent configuration removes cross talk between the wheels permitting one wheel to effectively track a positive obstacle while the other tracks a negative one. This factor combined with the lower un-sprung mass helps ensure that positive ground contact is maintained at all limes, with maximum isolation of the sprung mass from the road inputs.
Optimum ride quality is achieved for both types of suspension system by the use of soft springs and allowing large amounts of wheel travel. However, the handling characteristics of both types of suspension configuration suffer when soft springs are used because of excessive roll. The softer the suspension, the more the outer springs are compressed and the inner springs extended during a roll event. This rolling is due to weight transfer from the inside wheels to the outside wheels caused by the centripetal acceleration of the vehicle as it corners. This roll is accentuated by the displacement of the vehicle centre of gravity towards the outside wheels, an effect that is greatest in vehicles with high centres of gravity. Passive vehicle suspensions are thus often designed with auxiliary roll stiffness mechanisms such as anti-roll bars to attain an adequate compromise between ride, handling and control performance of the vehicle. Front-end “dive” under brake action is also more noticeable with soft springs.
The fact that axle cross talk is inherent to beam axles means that the introduction of an anti-roll bar has little detrimental effect on the ride quality and is seen to vastly improve the roll stiffness of the vehicle. The same however is not true for independent suspension systems. The key performance advantage for these is systems is the absence of axle cross talk and the complete independence of wheel movement over the full range travel. The introduction of any mechanical linkage across the axle restricts the independence of the system and introduces a cross-talk factor. This, while increasing the roll stability of the vehicle, produces a significant loss in ride quality, especially in challenging environments.
Vehicles fitted with pneumatic or hydro-pneumatic suspension systems offer the advantage of a variable spring rate, which can be manipulated to improve the roll stiffness of the vehicle. A number of systems have been proposed to improve the roll stiffness of vehicles by manipulating the pneumatic spring rate, all of which involve the cross linking of the spring elements on a given axle coupled with some means of oil transfer between them and/or external reservoirs of some description. See for example DE 4004204. Such systems again serve to detract from the independence of an independent suspension system, compromising on ride quality.
Japanese Patent Application Publication No. 62-286817 discloses a hydro-pneumatic suspension strut with a piston dividing a cylinder into a top chamber and a bottom chamber. Each chamber is connected through a damper with a separate accumulator. An external oil pump or accumulator connects to each chamber through in/out ports. This allows chamber pressure to be adjusted for attitude control. A normally closed shut valve connects between the chambers which, when opened, allows body height adjustment.
It is an object of the present invention to provide a vehicle suspension spring system that goes at least some way toward overcoming the above-mentioned problems and/or which will provide the public with a useful choice.
It is acknowledged that the term ‘comprise’ may, under varying jurisdictions be provided with either an exclusive or inclusive meaning. For the purpose of this specification, and unless otherwise noted explicitly, the term comprise shall have an inclusive meaning—i.e. that it may be taken to mean an inclusion of not only the listed components it directly references, but also other non-specified components. Accordingly, the term ‘comprise’ is to be attributed with as broad an interpretation as possible within any given jurisdiction and this rationale should also be used when the terms ‘comprised’ and/or ‘comprising’ are used.
Further aspects of the present invention will become apparent from the ensuing description which is given by way of example only.